Third and higher generation base station transmitters for applications, such as cellular telephone and other wireless communications typically use bandwidth-optimized modulation schemes for transmitting information. Bandwidth-optimized modulation schemes, such as CDMA, require a non-constant envelope and must operate a high peak-to-average ratio (PAR). Linear power amplifiers, such as class AB amplifiers are typically used in such systems because they offer high linearity, however, class AB amplifiers must be driven with a high back off, and, therefore, suffer from poor power efficiency. Class AB amplifiers must also be sized and biased to handle high peak power levels, even though they are often operated at much lower average powers. Currently, most state of the art base stations systems have a power efficiency of only 30%. This poor power efficiency results in high costs to build and operate base station systems.
The power efficiency of base station transmission systems can be improved by using alternative RF amplification systems and structures. In fact, power efficiency can be improved if a switched-mode amplifier, such as a class-F, an inverse class-F, or a class-D amplifier is used instead of a conventional class AB amplifier. A switched-mode amplifier is more efficient than a class AB amplifier because more of the amplifiers full dynamic range is utilized during operation and because current and voltage peaks do not occur simultaneously. In systems where bandwidth-efficient modulation schemes are required, such as in UMTS base stations, a special modulator must be developed to modulate both the time dependent phase and amplitude information in the time domain.
A number of prior art systems have been proposed and developed in order to utilize more power efficient amplifier architectures for bandwidth efficient modulation schemes. One solution, known as Envelope Elimination and Restoration (EER), separately modulates the phase and amplitude of the desired output. In EER, a constant envelope phase modulated signal is created along with a separate amplitude or envelope signal. The constant envelope phase modulated signal is then used to drive the input of a power amplifier, and the envelope signal is applied to adaptively change the bias or power supply of the power amplifier output stage. These bias or power supply regulation techniques, however, still suffer from poor power efficiency. Furthermore, EER becomes more difficult to implement and even more power inefficient as the modulation bandwidth increases. Wideband CDMA, for example has a modulation bandwidth of about 50 MHz, which is generally too high for the practical use of EER techniques.
Another solution to the power efficiency problem of bandwidth-efficient modulation is to use pulse-width modulation with a switched-mode power amplifier. Amplitude modulation is imparted at the amplifier output by varying the duty cycle of the pulse width modulated signal applied to the amplifier input. Pulse width modulation can be applied to a signal by using conventional pulse width modulation techniques (PWM) or by using delta-sigma modulation techniques. Delta Sigma modulation, however, tends to be less efficient then its PWM counterpart, which can offer power efficiencies of up to 60%.
PWM techniques, for all of its power efficiency advantages, suffer from a number of practical implementation difficulties. One difficulty involves the processing of RF pulses through a chain of RF amplifiers. Ideally, a PWM transmission system should comprise a very high bandwidth RF signal path system followed by a high order transmit filter. In reality, matching networks and bias networks limit the system's bandwidth and provide resonances that may demodulate and degrade the ideality of the PWM signals along the signal path. Generally, the more a PWM signal is degraded and departs from its constant envelope character, the lower the system's power efficiency and the worse the system's signal integrity becomes.
In the field of RF communication systems, what are needed are circuit system techniques for the effective implementation of PWM systems which implement bandwidth efficient modulation schemes.